Such accumulators can be called gas pressure accumulators, piston accumulators, diaphragm accumulators and bubble accumulators. Each accumulator has a predetermined pressure set value on the gas side before connection thereof to the fluid circuit. This value can be indicated as the initial gas pressure of the accumulator. In such accumulators, it is necessary at certain time intervals to monitor and control the initial gas pressure required for precise and safe operation. Some gas leakage is to be dealt with dependent upon the manner of operation of the accumulator. Furthermore, in safety devices, the gas level or gas pressure may not drop below a minimum level in an accumulator to guarantee that the system continues to operate.
German Patent No. 40 06 905 A1 (corresponding to U.S. patent application Ser. No. 07/664,285, entitled Method and Apparatus For Maintaining A Predetermined Theoretical Pressure Level and filed in the name of Karl-Eberhard Baldauf on Mar. 5, 1991, now abandoned, the subject matter of which is hereby incorporated by reference) discloses maintaining a predeterminable pressure set value in the accumulator by a measuring process in which at least at certain time intervals a connection for charging the gas is produced between the accumulator and a measuring chamber, having only a portion of the accumulator volume, while the measuring chamber is accessible through a suitable pressure measuring device. To maintain the predeterminable set pressure level when a pressure drop occurs, the connection is produced between the accumulator and a filling device through the measuring chamber. The measuring chamber then serves as a dosing chamber until the actual pressure level in the accumulator is again identical to the set value. High-cost and highly technical equipment is required for this known process for establishing gas pressure. Completely automatic operation is possible only with a control means of complicated construction. Thus, this known monitoring device is costly in construction and operation.
To limit the technical equipment outlay, a bubble accumulator is disclosed in the subsequently published German Patent No. 41 16 482 (corresponding to U.S. patent application Ser. No. 08/119,147, entitled Method of Measuring The Pressure Of A Gas In A Gas Accumulator And Device For Carrying Out The Method, and filed on Sep. 22, 1993 in the names of Gunther Peter and Norbert Weber, now U.S. Pat. No. 5,445,034, the subject matter of which is hereby incorporated by reference), to measure the gas pressure set value associated with the separating element (bubble accumulator) in a predeterminable position by means of a pressure pickup on the fluid side. The control of the position of the separating element occurs through the contact/closing position of the poppet valve of the accumulator. The contact position is controllable by means of a control device. However, considerable technical outlay is required for each individual accumulator to be controlled. Furthermore, the measuring device required for processing of the position data of the poppet valve and to pick up the values from the pressure level pick-up on the fluid side is technically costly. Thus, this monitoring device is costly in production and operation.
German Patent No. 26 49 049 A1 discloses a device for monitoring the position of a separating element movable within an accumulator in the form of a hydraulic cylinder. An ultrasound testing device is present in and arranged stationary on or near the bottom of the hydraulic cylinder. This known device detects movements and breakdown of the operation in underground mining operations, where the hydraulic cylinder being used must work safely and reliably for a long time under difficult working conditions. The measured path of movement of the separating element can also be detected from a remote station, so that central monitoring and control of an approximate point is possible. This device is costly and cost-intensive, since the controlled hydraulic cylinder is permanently provided with the ultrasound testing device to obtain monitoring or control. Additionally, the measuring and evaluating unit associated with the ultrasound testing device is correspondingly expensive, since the dynamic movements of the piston of the hydraulic cylinder are picked up by the monitoring system.
German Patent No. 37 32 219 A1 discloses a method in which electric-magnetic ultrasound conversion is used for control of filling level and bubble formation in enclosures (accumulators). The ultrasound testing device is mounted on the outside of the accumulator, and provides a coupling of the ultrasound test heads without liquid/fluid coupling medium and without special coupling foils. This known filling condition and bubble detection serves for monitoring/control of liquids in nuclear reactor pressure containers, especially those in the form of so-called boiling water reactors.